The Preliminary patch notes for 5.4 are out, and with them we know the names of the individual encounters in Eden's Promise.
Eden's Promise spoilers/thoughts, natch:
The first is Umbra, which makes sense given who the fight is against: The Cloud of Darkness. Though why we're fighting her is still a mystery at the moment.
The second is Litany. The core meaning of the word seems to be "a recital", suggesting that the fight is against a historical figure. It's against the big black wolf, of course—and given the location we've seen for that fight, it's pretty on the nose for a name.
But the third is where it gets weird. "Anamorphosis"—a distorted projection or drawing which appears normal when viewed from a particular point. The word means "Transformation" in its original Greek form, and is made of the root words "Ana" and "Morphosis" mean "Back, Again" and "A Shaping" respectively. I think this might be our Gaia/Mystery Antagonist fight.
EDIT 02: A shower thought. The primals we've faced in Eden, despite our intent to summon the originals, are all wildly warped from their original depictions. They're only really a match for the originals if viewed through a particular lens or from a particular point. I wonder if this fight might involve the Eden primals being reused in some capacity, as the trailer hinted that they would be returning in some capacity.
And the fourth is Eternity. Back when I posted my speculation on this, I believed Gaia and her "Fairy" would make up both the final encounters. I wonder if that might be true, and whatever we fight in "Eternity" is akin to Ultimecia's final form: Two alien women conjoined, their body "wearing"/devouring the universe.
Sad to say, there doesn't seem to be any room for Griever now. Aside from him and the final boss having a form resembling Ultimecia, I wasn't expect any further FF8 references. But it seems like there might not be any at all.
EDIT: Further thoughts, this time from lyrics!
Shadowbringers: "At this, the world's end, do we cast off tomorrow."
Return to Oblivion: "A voice from the past screaming 'there is no end'. A slave to my fate, ever doomed to repeat this (again and again and again)."
To the Edge: "Yes, time circles endlessly, the hands of fate train ahead (pointing to the edge)."
And two definitions of Eternity: "infinite or unending time." "a state to which time has no application; timelessness."
The latter definition definitely leans on the idea of "Time Compression"—all of time compressed to a single point. And it notably goes in the opposite direction of Gaia's character development: She goes from being lost in her own introverted misery to wanting to live to see "what tomorrow may bring". But in compressed time, only the here and now can exist. This does, at the very least, lend credence to the villain's modus operandi being the same as Ultimecia's.
The first definition, meanwhile, leans more on the emphasis of the conflict between Hydaelyn and Zodiark's followers being both endless, and invariably fatal, to both parties. I, um, seriously hope Gaia survives this. I actually like her character quite a bit, and find her friendship with Ryne to be sweet.
As an aside, I'm a bit surprised it took me this long to get the whole "time motif" Shadowbringers has had. It's pretty much everywhere, isn't it?